1. Field of the Invention
The invention relates to power converters, (source V-A/load V-A) of the DC-DC switchmode converter family. In particular, the invention relates to DC-DC switchmode converters of the compound type. More particularly, the invention relates to the differential combination of the boost (current sourced) topology and the reciprocal buck (voltage sourced) topology, to effect the new compound single quadrant DC-DC switchmode converter.
2. Description of the Prior Art
E. E. Landsman states that . . . "all three classical switching converter circuits" (FIGS. 1, 2, 3) "can be derived from a single canonical switching cell.", see E. E. Landsman, "A Unifying Derivation of Switching DC-DC Converter Topologies," PESC '79 Record (IEEE Power Electronics Specialists Conference--1979. (Publication #79CH1461-3 AES), June 18-22, 1979, p 243.
Peter Wood states that . . . "when we arrive at the single quadrant DC-DC converters, we find that the voltage sourced is the `buck` converter" (FIG. 2)"and the current sourced the `boost` " (FIG. 1)", reciprocals in every respect including transfer characteristics . . . the usual `buck-boost`" (FIG. 3)"is nothing more or less than a cascaded connection of boost and buck converters, . . .", see Peter Wood, "General Theory of Switching Power Converters", PESC '79 Record (IEEE Power Electronics Specialists Conference--1979, (Publication #79CH1461-3 AES), June 18-22, 1979, p 5.
Slobodan M. Cuk, et al, state that ". . . it has been found that the buck" (FIG. 2)", boost" (FIG. 1)"and buck-boost converters, previously considered to be a closed triad of simple power stages, are actually only three members of a four converter family. Completing the set is . . . the Cuk converter.", see Loman Rensink, Art Brown, Shi-ping Hsu, and Slobodan Cuk, "Design of a Kilowatt Off-Line Switcher Using a Cuk Converter," Proceedings of the Sixth National Solid-State Power Conversion Conference, May 2-4, 1979, p H3-2.
Robert D. Middlebrook and Slobodan M. Cuk state that ". . . general dc conversion . . . can be achieved by simply cascading the two basic converters, namely the boost" (FIG. 1) ". . . and the buck" (FIG. 2)". . . resulting in the same overall dc gain. . . While this converter" (FIG. 3)" has some good properties (both input and output currents continuous, that is, non-pulsating) it has some additional deficiencies. It needs an additional transistor Q2 and diode D2 which cause added dc and switching losses and hence significantly degrade the efficiency of the converter, besides its increase of complexity and number of components. Also at least one of the switching transistors requires floating drive circuitry, hence need for two isolated drive circuits, which further complicates its drive. Also there is no possibility to introduce the isolation property into this converter by simple means . . .", see Robert D. Middlebrook and Slobodan Cuk, U.S. Pat. No. 4,184,197, 1/1980, excerpted 3-63 through 4-18.
Since these prior art topologies have been so exhaustively analyzed, only the salient features (or lack thereof) will be discussed.
The boost converter (FIG. 1) displays continuous input current (current sourced), discontinuous output current, and the transfer function, E in/(1-.delta.)=E out.
The isolated boost converter (flyback) (FIG. 4) loses the continuous input current property, as the inductor L1 (FIG. 1) is incorporated into the flyback transformer T1 (FIG. 4).
The buck converter (FIG. 2) displays discontinuous input current (voltage sourced) continuous output current, and the transfer function E in(.delta.)=E out.
The isolated buck converter (forward) (FIG. 5) requires an additional transformer T2 and diode D3.
The cascade boost-buck converter (FIG. 3) displays continuous input current, continuous output current, and the transfer function E out=.delta.(E in+E out), thus realizing the general DC-DC conversion function. The boost-buck transfer function may be clarified by setting E in (FIG. 1)=a, E out (FIG. 1)=E in (FIG. 2)=b, E out (FIG. 2)=x, t on/T=.delta.. Substituting and transposing [a/(1-.delta.)=b](boost), [b(.delta.)=x] (buck), then [x=.delta.(a+x)] (boost-buck). Given 0&lt;.delta.&lt;1, and 0&lt;a&lt;.infin. then x may be derived from differential control of .delta..
The isolated Cuk converter (FIG. 6) realizes the general DC-DC conversion function, in compound topology. However, this series capacitance fed, coupled inductor topology exhibits several undesirable properties. These negative properties include
(1) output voltage reversal at turn-on; G. E. Bloom, A. Eris, and R. Ruble state that "one undesirable feature of operation . . . , namely that of transient voltage polarity reversal . . . must be circumvented or reduced to acceptable magnitudes."; see G. E. Bloom, A. Eris, and R. Ruble, "Modeling, Analysis, and Design of a Multi-Output Cuk Converter," Proceedings of Powercon 7, Mar. 24-27, 1980, p 11-14.
(2) requirement for power component damping; Alan Cocconi and Slobodan Cuk state that ". . . one must find the method which will introduce the required damping . . . to damp otherwise unacceptable high resonant peaks of the pole pairs."; see Alan Cocconi and Slobodan Cuk, "Design of a 2 KW, 100 KHZ Switching Regulator for Space Shuttle", Powerconversion International, January 1983, p 14-15.
(3) right half plane zero; Alan Coccini and Slobodan Cuk state that ". . . frequency response contains a very nasty right half plane zero . . . immune to all attempts of passively damping . . . "; see pages 20-21 of the last-mentioned reference.
(4) topological inefficiency; this series capacitor fed configuration requires that both primary and secondary of T1 conduct continuously, i.e., during both the energy storage cycle (Q1 off), and the energy delivery cycle (Q1 on). At .delta.=0.5 duty cycle, this involvement doubles the resistive losses, according to the formula i rms=.sqroot.0.5i.sup.2 +0.5i.sup.2. The second 0.5i.sup.2 term disappears from the conventional forward transformer loss equation. Additionally, the unterminated reactance (leakage inductance) of T1 contributes doubly to the damping losses of (2);
(5) additional safety burden; the "floating" (ungrounded) case installation of C1 and C2 (FIG. 6) imposes insulation/safety considerations not found in parallel (grounded) capacitor topologies;
(6) complex loop compensation requirements; Alan Cocconi and Slobodan Cuk state that "all attempts to close the feedback loop by conventional means, . . . are either futile, or result in . . . unusable transient responses, far away from required specifications", see pages 20-21 of the last-mentioned reference.
The cascade boost-buck topology (FIG. 3) is seen to realize the idealized general DC-DC conversion function. If the seemingly intractable deficiencies previously cited could be overcome (inefficiency, complexity, impossibility of simple isolation, etc.), the cascade boost-buck topology (FIG. 3) would be the preferred topology in single-quadrant DC-DC conversion.
The foregoing suggests that an ideal source V-A/load V-A converter should incorporate at least the following set of objectives:
it should realize the idealized general DC-DC conversion function;
it should provide for intrinsic circuit response to source/load demands, extraordinary to feedback loop parameters;
it should be intrinsically stable without resort to power dissipating damping;
it should exhibit theoretically infinite source/load voltage range;
it should require only first-order feedback loop compensation and minimum gain bandwidth;
it should deploy both input voltage and load current feed-forward by topologically inherent function, in current mode loop control;
it should function in both the continuous and discontinuous states of internal current flow;
it should obtain multiple, isolated, and stable output voltages without resort to minimum loads, pre-loads, sub-regulation, or other circuit manipulation.
it should achieve galvanic isolation between output voltages, as well as between input and output voltages;
it should eliminate output inductor saturation as induced by volt-second unbalance during overload and short circuit;
it should demonstrate continuous, non-pulsating, input and output currents;
it should suffer no power losses, safety constraints, or polarity anomalies in consequence of the topologically inherent deficiencies characteristic of prior art;
it should be capable of realization with readily available materials and components, requiring no "exotic" or yet-to-be-perfected apparatus;
it should exceed the composite power/performance density of all prior circuit art in the field of the invention.
it should introduce an entirely new topology to the converter family, a fifth and penultimate member of the set.